The christening dress here identifies the occasion.
In the normal course of events, Victorian couples would produce their first baby within a year or so of the wedding. Christening usually took place anything from 4 to 8 weeks after the birth.
In rich Victorian families christenings could be elaborate affairs emphasizing the high social rank into which the infant was born. The christening robe itself could represent a lavish statement of family fortunes. Great care was usually taken to display the robe to best advantage in the portrait.
New babies were pictured with their mothers and in the case of wealthy families with their nurses. Nurses often wore uniforms, but not always. So we must not assume automatically that the woman with the baby was its mother.